Actuators are relatively simple mechanical components that are often incorporated into more complex mechanical systems, including those found in automobiles, airplanes, manufacturing facilities, and processing facilities. A conventional solenoid is one example of an actuator that has found broad application across many types of industries and technologies.
Shape memory alloys (SMAs) are metals that exist in two distinct solid phases, referred to as Martensite and Austenite. Martensite is relatively soft and easily deformed, whereas Austenite is relatively stronger and less easily deformed. SMAs can be induced to change phase by changes in temperature and changes in mechanical stress. SMAs can generate relatively large forces (when resistance is encountered during their phase transformation), and can exhibit relatively large movements as they recover from large strains. SMAs have been used commercially in many types of actuators, where a temperature change is used to control the actuation cycle. One of the most widely recognizable applications has been the use of SMA based actuators in automatic sprinkler systems.
One disadvantage of SMA actuators triggered by changes in temperature is that, depending upon the application, a heating or cooling device may need to be incorporated into the actuator, increasing the size, expense, and complexity of the actuator. Further, the response of such an actuator depends on heat transfer, which can occur too slowly for certain applications. Material scientists have more recently recognized that the phase change between Martensite and Austenite can be induced by changes in an applied magnetic field in certain alloys, as well as by changes in temperature and stress loading. Because magnetic fields generated with electromagnets can be rapidly switched on and off, particularly as compared to the time required to induce a change in temperature to initiate an actuation, electromagnetically controlled SMA based actuators appear to offer promise in applications where rapidly responding actuation is required. Such alloys are referred to as ferromagnetic shape memory alloys (FSMAs).
One such type of FSMA is an alloy of iron and palladium (FePd). Unfortunately, the cost of palladium is so prohibitive that commercial utilization of FePd-based actuators is not now economically feasible. In an attempt to identify other materials that could be of use in FSMA actuators, composites of a ferromagnetic material and a SMA alloy that itself is not ferromagnetic have been suggested. Because of the widespread utility of actuators, it would be desirable to develop different embodiments of actuators incorporating FSMAs (or composite FSMAs) that will likely have commercial value.